Structural engineers are baffled by what might have caused the roof collapse at Paris' Charles de Gaulle-Roissy airport, which killed four people.

The ultra-modern Terminal 2E, which opened 11 months ago, was built using steel, concrete and 36,000 sq m (118,116 sq ft) of reinforced glass.

The futuristic building was based on ideas learned in tunnel construction

It consists of two long tunnel-like buildings connected by a central passageway.

The part that collapsed was a 30-metre (98-foot) section of roof on the outer building, where passengers wait before boarding the aircraft.

The investigation into what went wrong will focus primarily on whether there was a fault in the design of the building, or whether short cuts had been taken when it was built.

The internationally-renowned French architect who designed the terminal, Paul Andreu, has described the structure as "bold" but "nothing revolutionary".

The structure stood for a year and then spontaneously fell down without anything apparently changing. It's very odd.

Chris WiseStructural engineer

Buildings like that do not just collapse, leading structural engineer Chris Wise told BBC News Online.

"Something very unusual has happened," he said.

"The structure stood for a year and then spontaneously fell down without anything apparently changing. It's very odd.

"It's not an unusual structure. There are thousand of structures of similar size across the world.

"People have been building structures that size since Roman times."

Materials

There are various theories as to what might have caused the collapse, some of which have been discussed on various online forums. But none of them leap out, Professor Wise said.

Some speculation focuses on a flaw in the materials - possibly in the concrete used to make up a series of interlocking rings that form the basis for the tube-like structure.

But Professor Wise said that was unlikely. The materials would have been checked at the factory, and then again at the site - and anyway, if the concrete was the problem, the building would probably have fallen straight away.

Similarly, if there was a problem with the joints, it would normally be obvious, he said.

Another theory is that the ground on which the terminal was built may settled, causing the structure to shift.

"It can take up to 10 years for the ground to move," he said. "But the building design would have taken that into account."

So could a mistake have slipped through?

There are lots of checks and balances in a project like this, Professor Wise said.

"If you make a mistake with the design there's almost no chance of your design getting built," he said. "Somebody along the way would find it."

Deliberate collapse?

Sabotage was quickly ruled out by officials as the cause of the roof collapse.

But for Professor Wise, sabotage remains a possible explanation.

"Things don't just fall down like that," he said. "Conspiracy theorists would say that if it was sabotage, the airport could never admit that."

Another possible theory to look into, he said, would be the effect of temperature, and whether the early morning temperature at the time of the collapse had somehow contributed.

Concrete and steel heat up and expand at different rates - but again, this would have been taken into account.

Wobbly bridge

The people that worked on the project are "exceptionally talented and have proved themselves time and time again", Professor Wise said.

Chris Wise is the director of Expedition Engineering and also teaches at Imperial College London.

He worked on London's Millennium Bridge, the famous pedestrian walkway over The Thames that developed a sideways wobble soon after its opening in June 2000 - proving that unforeseen problems can occur in the best-planned project.

Some kind of anomaly could yet explain the Paris roof collapse.

"There may be some inherent problem that no-one knows about," he said

Whatever the cause might be, French Prime Minister Jean-Pierre Raffarin has said that investigations "will bring out the truth."